Recent revelations will have made unsettling reading for those who still believe in Britain's essentially benign approach to world affairs: evidence of British collusion with loyalist paramilitary groups in Northern Ireland engaging in "targeted assassinations" of suspected IRA members, for example, and the mounting anger over the way in which the government not only doctored intelligence reports on weapons of mass destruction, but also misled the House of Commons, and indeed the whole country, over the nature of the threat posed by Saddam Hussein.

In his brilliant and deeply disturbing new book, Mark Curtis demonstrates that these cannot be brushed aside as isolated cases, and delivers a powerful challenge to the notion that Britain's foreign policy is basically benevolent: that it promotes democracy, peace and human rights. The truth, according to Curtis, is that Britain supports terrorism. Indeed, "violating international law has become as British as afternoon tea".

But if New Labour has raised hypocrisy and double standards to the level of a new art form, ("never in history has there been such a gap between government claims and the reality of policy"), the route they are pursuing is hardly a new one. Curtis convincingly shows that contempt for international law has passed easily from Conservative to Labour, and from the colonial era to the present. By imposing regime change in Iraq, for example, Tony Blair is not so much following the US as continuing a national tradition.

And Curtis marshals a formidable armoury of documented evidence to substantiate his case. Drawing on formerly secret government files, he analyses not only Britain's role in recent events in Kosovo, Afghanistan and Iraq, but also British complicity in the slaughter of a million people in Indonesia in 1965; the depopulation of the island of Diego Garcia; the overthrow of governments in Iran and British Guiana; and repressive colonial policies in Kenya, Malaya and Oman. He relentlessly peels away layers of deception until, with the aid of painstaking research and analysis of declassified files, he lays bare in graphic detail a shocking exposé of British aggression and double-standards.

It could scarcely be more timely. A joke circulating during the war on Iraq was that the reason British government ministers are so sure that Saddam had WMD is simple: they still have the receipts. In Curtis's chapter on Iraq, those receipts are documented in a devastating analysis of Britain's systematic arming of Saddam Hussein. Just five months after the poison gas attack on Halabja in 1988, the foreign secretary, Geoffrey Howe, noted in a secret report to Margaret Thatcher that, with the Iran-Iraq peace deal agreed in August, "opportunities for sales of defence equipment to Iran and Iraq will be considerable". The secrecy of this policy was vital, since, as one Foreign Office official noted, "it could look very cynical if, so soon after expressing outrage about the treatment of the Kurds [at Halabja], we adopt a more flexible approach to arms sales".

But dealing with concern about "looking cynical" is something the proponents of British foreign policy learned as far back as 1953, during the forgotten British invasion of British Guiana. "To secure the desired result, some preparation of public opinion seems to be essential," noted the British delegation to the UN. Democratic elections had resulted in victory for a popular, leftist government committed to reducing poverty. Its plans threatened the British sugar multinational, Bookers, which pleaded with London to intervene. Britain dispatched warships and 700 troops to overthrow the government, ruling out elections on the grounds that "the same party would have been elected again", according to the colonial secretary.

Curtis's thesis, that people are systematically misinformed about this country's real role in the world, reaches a powerful climax with his analysis of Britain's role in the 1994 Rwanda genocide. From official files, he describes how the British government "used its diplomatic weight to reduce severely a UN force that, according to military officers on the ground, could have prevented the killings", and, in late April 1994, along with the US and China, secured a security council resolution that rejected the use of the term "genocide", so that the UN would not act.

If, as Curtis demonstrates, the government's contribution to something as massive as the slaughter of a million people can be buried across the entire mainstream political culture, he is surely right to conclude that anyone who wants to understand the reality of Britain's foreign policies cannot do so by relying on the mainstream. "The task of any independent historian is to reconstruct real-life history, to rescue it from a self-serving web of deceit," he asserts. And indeed Curtis has performed a remarkable rescue operation. His book is a powerful call to action for all those who strive to understand how the world has been shaped by western powers in order that they may change it.